When a fan is mounted to the permanent magnet synchronous motor for the purpose of air blowing, the motor may be considered to run free due to the air blowing. The same may be said with a pump and to smoothly restart the free-running permanent magnet synchronous motor, it is necessary to detect the rotor phase and rotation speed during free running. However, direct detection of the rotor phase and rotation speed by a position sensor causes an increase in cost in correspondence to the position sensor. Therefore, to decrease the cost, an art for inferring the rotor phase and rotation speed of the free-running permanent magnet synchronous motor without using the position sensor is proposed.
For example, the method described in Patent Document 1 may be cited. When the electromotive force (EMF) generated in the free-running permanent magnet synchronous motor is impressed to the output terminal of the inverter, the two inter-line voltages are compared with each other and the rotor phase and rotation speed are inferred from the magnitude of the difference between them. By doing this, the permanent magnet synchronous motor can be restarted continuously from the free run condition without stopping. However, to make a comparison between the inter-line voltages, a comparator is required and a problem arises that the cost and mounting area increase.
Furthermore, the method described in Patent Document 2 may be cited. The output terminal voltage of the inverter is measured using an A/D port and the rotor phase and rotation speed are inferred from changes in the measured values. To measure the three-phase output terminal voltage, three A/D ports are necessary originally and a means for realizing it only by one A/D port is proposed. However, instead of it, a transistor and an I/O port must be used and the transistor must be switched to ON or OFF at high speed.